What a cooling system does and its main parts
A cooling system removes heat from inside your home and moves it outside. The most common type is central air conditioning, which uses a network of ducts to distribute cold air throughout the house. The system has four essential parts: the outdoor unit (condenser), the indoor unit (evaporator coil), the air handler, and the thermostat that controls when the system runs.
When you set your thermostat to a temperature, the system pulls warm air from inside your home through return ducts. That air passes over the cold evaporator coil, which absorbs the heat. The cooled air then flows back into your rooms through supply ducts. Meanwhile, the heat that was removed travels to the outdoor unit, where it is released into the air outside.
A refrigerant — a special fluid that changes between liquid and gas — circulates through the system and does the actual work of moving heat. The compressor in the outdoor unit pressurizes this refrigerant, and the condenser fan helps release the heat to the outside air. Without all four parts working together, the system cannot cool your home.
Key Takeaways
- Central air conditioning moves heat from inside your home to the outside by circulating refrigerant through an indoor coil and an outdoor unit.
- The thermostat signals when cooling is needed, the indoor coil cools the air, and the outdoor unit releases the heat outside.
- A clogged air filter, low refrigerant, or a broken compressor are common reasons a system stops cooling effectively.
- Changing the air filter every one to three months and having the system serviced once a year can prevent many problems.
- If your system is not cooling, leaking water, or making unusual noise, contact a licensed HVAC technician rather than attempting repairs yourself.
How the cooling cycle works step by step
The cooling process repeats continuously while your thermostat calls for air conditioning. First, warm air from your home is drawn into the return ducts and passes through an air filter that traps dust and debris. This filtered air then flows over the cold evaporator coil, where refrigerant absorbs the heat and cools the air to the temperature you set.
The cooled air is pushed by the air handler through supply ducts and out into your rooms through vents and registers. At the same time, the refrigerant that absorbed the heat travels to the outdoor unit as a gas. The compressor pressurizes this gas, which makes it even hotter. The hot, pressurized gas then flows through the condenser coil, where a fan blows outside air across it to release the heat to the environment.
Once the refrigerant has released its heat, it cools back down and returns to the indoor coil as a liquid, ready to absorb more heat. This cycle repeats many times per hour. The thermostat monitors the indoor temperature and signals the system to stop when the desired temperature is reached, then restarts it when the temperature rises again.
Common reasons a cooling system stops working or cools poorly
A clogged air filter is the most frequent cause of reduced cooling. When the filter becomes blocked with dust, airflow slows and the evaporator coil can freeze over, stopping the system from cooling at all. Checking and changing the filter every one to three months — more often if you have pets or live in a dusty area — prevents this problem.
Low refrigerant is another common issue. If the system has a leak, the refrigerant level drops and the coil cannot absorb enough heat. You will notice the air coming from your vents is not as cold as it should be, or the system runs constantly without reaching the set temperature. A licensed HVAC technician must locate and repair the leak, then refill the refrigerant.
A broken compressor, a faulty thermostat, or a failed capacitor (a component that helps start the outdoor unit) can also stop cooling. If the outdoor unit is not running at all, the problem is often electrical. If the outdoor unit runs but no cold air comes from your vents, the indoor coil or air handler may be the issue. Water pooling around the indoor unit usually means the condensate drain line is clogged.
Maintenance tasks you can do yourself
The most important task is changing the air filter. Locate the filter in the return air duct or in a filter rack near the air handler — usually in a basement, attic, or utility closet. Note the filter size (printed on the frame, such as 16x25x1) and buy a replacement at a hardware store. Slide out the old filter, note which direction the arrow on the new filter points, and insert it so the arrow points toward the air handler.
You can also keep the outdoor unit clear of debris. Leaves, grass clippings, and dirt reduce airflow and make the system work harder. Turn off the system at the thermostat, then gently remove any visible debris from around the outdoor unit. Do not use a pressure washer, as the force can damage the fins. If the fins are bent, a technician can straighten them, but this is not urgent.
Check that all vents and registers inside your home are open and not blocked by furniture or curtains. Blocked vents reduce airflow and make the system less efficient. You can also visually inspect the ductwork in your attic or basement for obvious gaps or disconnections, though sealing ducts properly requires special equipment and should be done by a professional.
When to call an HVAC technician
Contact a licensed HVAC technician if your system is not cooling, even after you have changed the air filter. Also call if the system is running but the air coming from your vents is not cold, if the outdoor unit is not running, or if you hear unusual noises like grinding, squealing, or rattling. Water leaking from the indoor unit or pooling around it also requires professional attention.
If your system is more than 10 to 15 years old and breaks down, ask the technician whether repair or replacement makes sense. Older systems become less efficient and more expensive to fix. A technician can tell you the expected lifespan of your specific unit and the cost of a new system if replacement is recommended.
Schedule a professional maintenance visit once a year, ideally in spring before the cooling season begins. During this visit, the technician will clean the coils, check the refrigerant level, test the thermostat, inspect electrical connections, and may support all components are working properly. This prevents many problems and extends the life of your system.
Understanding your thermostat settings
Most thermostats have a mode selector with options like "Cool," "Heat," "Auto," and "Off." Set it to "Cool" during warm months to run only the air conditioning. "Auto" mode switches between heating and cooling depending on the temperature, which is useful during spring and fall when you might need both on the same day. "Off" stops the system entirely.
The fan setting usually has options for "Auto" and "On." In "Auto" mode, the fan runs only when the system is actively cooling. In "On" mode, the fan runs continuously, which can help circulate air but uses more electricity. For most homes, "Auto" is the efficient choice.
Programmable and smart thermostats let you set different temperatures for different times of day. Raising the temperature by a few degrees when you are away or sleeping reduces energy use without sacrificing comfort. Many people find that a setting of 78°F during the day and 82°F at night saves money while keeping the home reasonably cool.
How to prepare your system for the cooling season
In early spring, before you turn on the air conditioning for the first time, change the air filter and visually inspect the outdoor unit for winter damage or debris. Turn the thermostat to "Cool" and set it a few degrees below the current indoor temperature to test that the system starts and cold air flows from your vents within a few minutes.
If the system does not start, check that the outdoor unit has power — look for a disconnect switch (usually a small box near the unit) and make sure it is in the "On" position. If the unit still does not run, turn off the thermostat and call a technician rather than troubleshooting further.
If you have not had the system serviced in the past year, schedule a maintenance visit. The technician will may support everything is ready for heavy use during summer and can catch small problems before they become expensive repairs.
Frequently Asked Questions
Why is my air conditioner running but not cooling the house?
The most common cause is a clogged air filter, which blocks airflow and can freeze the indoor coil. Change the filter first. If that does not help, the refrigerant may be low due to a leak, or the outdoor unit may not be running. Check that the outdoor disconnect switch is on and that the outdoor unit fan is spinning. If these are fine, call a technician.
How often should I change my air filter?
Change it every one to three months during cooling season. If you have pets, allergies, or live in a dusty area, change it monthly. A clogged filter reduces cooling and increases energy use, so checking it monthly and replacing it when it looks dirty is the best approach.
What does it mean if water is pooling around my indoor unit?
The system produces condensation as it cools the air, and this water normally drains through a condensate line to a floor drain or outside. If water is pooling, the drain line is likely clogged with algae or debris. A technician can clear it, or you can try pouring a cup of bleach down the drain line opening if you can locate it safely.
Is it normal for my outdoor unit to be hot?
Yes. The outdoor unit releases the heat that was removed from your home, so it will be noticeably warm or hot to the touch. However, if it is so hot that you cannot hold your hand near it for more than a second, or if it is leaking fluid, contact a technician.
How much does it cost to repair or replace a cooling system?
Repair costs vary widely depending on what is broken. A straightforward fix like clearing a drain line might cost $100 to $200, while replacing a compressor can cost $1,000 to $2,500. A new system typically costs $3,000 to $7,000 or more depending on the size of your home and the efficiency rating. Get quotes from at least two technicians before deciding.